Many manufacturers are competing to develop an electroluminescence element (OLED element) made of an organic material for an application such as a thin and space-saving illumination or a display.
The greatest challenge is to improve the external light extraction efficiency of the element. This is because more than half of the light emitted from inside the element cannot be output to the outside of the element due to the effect that the emitted light is reflected by a transparent electrode or the like having a high refraction index in the middle of exiting the element or other such effects. To address this problem, there have been proposed several approaches of providing various layers (light extraction efficiency improving layers) for improving the light extraction efficiency to the transparent electrode or the like.
Patent Literatures 1 and 2 propose improving the light extraction efficiency by providing on the transparent electrode a layer in which particles are dispersed in a binder, the particles having a refraction index that is different from that of the binder. Further, Non Patent Literature 1 proposes improving the light extraction efficiency by providing a layer formed of a titanium oxide thin film having voids therein.
In Patent Literatures 1 and 2, the binder layer containing the particles involves an organic solvent and is thus formed by, instead of a vacuum process, preparing a mixture of the binder, the powders, and the organic solvent, and applying and drying the mixture under normal pressure. In Non Patent Literature 1, the layer is also formed on the transparent electrode under normal pressure by spin coating. In Patent Literatures 1 and 2, and Non Patent Literature described above, the light extraction efficiency improving layer is provided on the transparent electrode.